The invention relates generally to the assembly of components onto a surface, and more particularly, to the assembly of building blocks onto a substrate for electronic circuit fabrication, sensors, energy conversion, photonics and other applications.
There is a concerted effort to develop large area, high performance electronics for applications such as medical imaging, nondestructive testing, industrial inspection, security, displays, lighting and photovoltaics, among others. Two approaches are typically employed. For systems involving large numbers of active elements (for example, transistors) clustered at a relatively small number of locations, a “pick and place” technique is typically employed, for which the active elements are fabricated, for example using single crystal semiconductor wafers, and singulated (separated) into relatively large components (for example, on the order of 5 mm) comprising multiple active elements. The components are sequentially placed on a printed circuit board (PCB). Typically, the components are sequentially positioned on the PCB using robotics. Because the pick and place approach can leverage high performance active elements, it is suitable for fabricating high performance electronics.
A key limitation of the pick and place approach is that the components must be serially placed on the PCB. Therefore, as the number of components to be assembled increases, the manufacturing cost increases to the point where costs become prohibitive. In addition, as the component size decreases, it becomes increasingly difficult to manipulate and position the components using robotics. Accordingly, this technique is ill-suited for the manufacture of low density, distributed electronics, such as flat panel displays or digital x-ray detectors. Instead, a wide-area, thin film transistor (TFT) based approach is typically employed to manufacture low density, distributed electronics. Typically, the TFTs comprise amorphous silicon (a-Si) TFTs fabricated on large glass substrates. Although a-Si TFTs have been successfully fabricated over large areas (e.g. liquid crystal displays), the transistor performance is relatively low and therefore limited to simple switches. In addition, with this process, the unit cost of a large area electronic circuit necessarily scales with the size of the circuit.
Another approach is to substitute a higher mobility semiconducting material, such as polysilicon, cadmium selenide (CdSe), cadmium sulfide (CdS) or germanium (Ge), for a-Si to form higher mobility TFTs. While TFTs formed using these higher mobility materials have been shown to be useful for small-scale circuits, their transistor characteristics are inferior to single crystal transistors, and thus circuits made from these materials are inherently inferior to their single crystal counterparts. As with a-Si, the unit cost of a large area electronic circuit necessarily scales with the size of the circuit, for this process.
A number of approaches have been developed to overcome these problems. For example, U.S. Pat. No. 5,783,856, to Smith et al., entitled “Method for fabricating self-assembling microstructures,” employs a fluidic self-assembly process to assemble trapezoidal shaped components dispersed in a solution onto a substrate having corresponding trapezoidal indentations. This approach uses gravity and convective fluid flow to deposit the components in the indentions. Limitations of this technique include: the use of relatively weak forces to dispose and hold the blocks in the indentations. It would further appear to be difficult to assemble a large variety of elements to the substrate due to the limited number of block and indent shapes that can realistically be fabricated.
U.S. Pat. No. 6,657,289, to Craig et al., entitled “Apparatus relating to block configurations and fluidic self assembly process,” employs a fluidic self-assembly process to assemble components having at least one asymmetric feature dispersed in a solution onto a substrate having correspondingly shaped receptor sites. Limitations of this technique include: the use of relatively weak forces to dispose and hold the blocks in the shaped sites. It would further appear to be difficult to assemble a large variety of elements to the substrate due to the finite number of component shapes available.
U.S. Pat. No. 6,780,696, to Schatz, entitled “Method and apparatus for self-assembly of functional blocks on a substrate facilitated by electrode pairs,” employs another fluidic self-assembly process to assemble trapezoidal shaped components dispersed in a solution onto a substrate having corresponding trapezoidal indentations. However, this approach couples electrodes to the substrate to form an electric field. The approach further forms the components of high-dielectric constant materials, such that the components are attracted to higher electric field regions and are thus guided to the trapezoidal indents. In another embodiment, the component is formed of a low magnetic permeability material, and a high magnetic permeability layer is coupled to the bottom surface of the component. A static magnetic field is generated at a receptor site by covering the receptor site with a permanent magnet having a north and a south pole aligned such that the static magnetic field is aligned parallel to the surface of the receptor site. In another embodiment, a magnetic field is applied parallel to the substrate. The slurry solution has an intermediate value of magnetic permeability. A drawback of this technique is that the components will tend to agglomerate in solution, due to the propensity of high magnetic permeability materials to agglomerate so as to minimize magnetic energy. Another possible limitation on this technique is registration error between the component and the substrate resulting from the use of magnetic fields aligned parallel to the substrate. In addition this technique would not lend itself to the assembly of multiple component types.
U.S. Pat. No. 3,439,416, to Yando, entitled “Method and apparatus for fabricating an array of discrete elements,” forms pairs of magnets in a laminated base. Magnetic coatings, such as iron, are applied to the surface of elements. A multiplicity of elements is placed on the surface of the laminated base, which is then vibrated to move the elements. The magnetic coated surfaces of the elements are attracted to the pole faces of the magnet pairs. This technique suffers from several drawbacks, including severe limitations on the shape, size and distribution of the elements. For example, element width must match the spacing of the magnetic layers in the laminated base and the distribution of the elements is restricted by the parallel lamination geometry. In addition the technique appears to be applicable to relatively large, millimeter sized dimensions, and may not be suitable for smaller, micron-sized elements. In addition this technique would not lend itself to the assembly of multiple component types.
“Programmable assembly of heterogeneous colloidal particle arrays,” Yellen et al., Adv. Mater. 2004, 16, No. 2, January 16, p. 111-115, employs magnetically programmable assembly to form heterogeneous colloidal particle arrays. This approach utilizes micromagnets that are covered with an array of square microwells and which are magnetized parallel to the plane. The substrate is immersed in a bath, and superparamagnetic colloidal beads are injected into the bath. External magnetic fields are applied perpendicular to the plane in a first direction, causing the beads to be attracted to one pole of the micromagnets. The direction of the external magnetic field is then reversed, causing the beads to be attracted to the other pole of the micromagnets. A drawback of this technique is that it is limited to two types of particles. Another limitation of this technique is that it requires the application of external magnetic fields and appears to be limited to superparamagnetic colloidal beads. Another limitation on this technique is use of microwells to trap the beads. Yield would also appear to be an issue.
It would therefore be desirable to provide systems and methods for fabricating high performance, large area electronics rapidly and inexpensively. It would further be desirable for the improved systems and methods to facilitate the assembly of a variety of different types of elements.